A virtual machine monitor (“VMM”) creates an environment that allows multiple operating systems to run simultaneously on the same computer hardware. In such an environment, applications written for different operating systems (e.g., Windows, Linux) can be run simultaneously on the same hardware.
The VMM is loaded during bootup of the computer and receives control of the hardware at boot time. The VMM maintains hardware control until the computer is shut down.
Running an operating system on a VMM involves virtualizing one or more of the following types of hardware at bootup: memory, I/O devices, and CPU(s). CPUs can be virtualized in the traditional way: the hardware is configured to trap when an operating system (“OS”) executes a privileged instruction, and the VMM simulates that instruction to maintain the illusion that the operating system has sole control of the hardware. To virtualize memory, the VMM typically manages memory translation in order to translate between the OS's use of physical memory, and the real “machine” memory present in hardware.
I/O virtualization typically involves two levels of device drivers, one in the OS, and one in the VMM. Drivers in each level provide their own interrupt handlers. In some systems, OS device drivers are designed with knowledge of the VMM drivers. In such systems, the OS device driver performs I/O by directly calling routines in the VMM driver. In other systems, the OS device driver is unaware of the VMM. In those other systems, the driver in the OS attempts to perform I/O as if that driver controlled the I/O device, but the VMM intercepts those attempts and instead performs the I/O on that driver's behalf, using the VMM device driver. I/O performed in this manner is called “emulated I/O.” When interrupts from an I/O device occur, interrupt handlers in the VMM drivers get control first. They process the interrupt, and then, as needed, deliver the interrupt to the appropriate handler among the OS's device drivers.
Virtualizing the hardware adds overhead to the computer. For example, intercepting the I/O instructions performed by the OS drivers and using two levels of interrupt handlers slow the OS's normal I/O processing.
Since the VMM virtualizes the hardware from bootup to shutdown, overhead is incurred even when virtualization is not necessary (for example, when only a single OS instance is running on the hardware). Thus the VMM can add unnecessary overhead to the computer.
It would be desirable to reduce the unnecessary overhead.